Such differences of size and form indicate that these spi- 
cules may have belonged to several species of sponge and 
this is rendered further probable by the great numbers which 
are present in this Horstead flint. 
I have some difficulty in determining the relationship of 
this group of spicules with existing forms. The compound 
trifid spicule with the head expanded horizontally frequently 
forms the «zone» spicule of Geodia and Stelletta, but accom- 
panied by an elongated shaft; a similar form of spicule is 
present in the dermal portion of many species of Lithistids, 
but these are so diminutive in comparison with those figured 
as to negative the probability of their being surface spicules of 
these sponges; I also compared them with the truncated spicules of 
Pachastrella, but these have the arms mostly simple, or where bifur- 
cated, the division is generally restricted to the ends of the arms; at 
last I found the description of a sponge with a similar spicule, 
placed by Mr. Carter under the name of 77s¢phonia nana(An. Mag. 
Nat. Hist. S. 5, Vol. 6, p. 492, Pl. VII, fig. 43 a, b). This spicule 
has a truncated shaft and an expanded head closely resem- 
bling some of the Horstead forms. Further, O. Schmidt has 
figured the head ofa spicule similar to that of fig. 20 in a 
sponge from the Adriatic under the name of Stelletta Hilleri 
(Supp. d. Spong. Adriat. Meeres, p. 32, Taf. III, fig. 8) and 
as according to Mr. Carter the genus Stelletta is equivalent 
to Tisiphonia, there is a probability that these spicules with 
truncated shafts may be the «zone» spicules ot sponges allied 
to Stelletta, notwithstanding the great differences which they 
present, in the comparative reduction of the shaft and the 
extended development ot the head, from the typical «zone» 
spicules of Stelletta itself. 
Similar spicules but as a rule very much smaller in di- 
mensions, have been found by Mr. Carter in the Haldon 
Green Sand (An. Mag. Nat. Hist., S. 4, Vol. 7, Pl. IX, 
figs. 30—36); they are also present in the Chalk of the 
North of Ireland (Wright op. cit. Pl. Il, figs. 17 a, b) and 
